Basement-controlled multiple slope collapses, Rockall Bank Slide Complex, NE Atlantic

The eastern flank of the Rockall Bank is dominated by the scarps of a large slide complex with unusual low aspect ratio (120 km wide and 150 km long). With the use of an extensive 2D seismic dataset, re-processed high-resolution multibeam bathymetry, a TOBI mosaic and one piston core we demonstrate that the headwall of the Rockall Bank Slide Complex (previously referred to as the Rockall Bank Mass Flow) may have occurred as a multiphase slope collapse in at least three episodes of instability since 16 ka BP, with each of the events generating a slide with a more typical elongate morphology. Reconstructions of the pre-slide seafloor suggest a volume of collapsed sediments in the range265–765 km3. Seismic facies investigation indicates that the slope and failed sediments comprise almost exclusively contourites that are prone to failure due to their well sorted character and high water content. A systematic examination of the slope morphology, seismic facies and sub-seafloor structures demonstrates a direct relationship between the RBSC seafloor scarps and the basement morphology. Focused fluid flow along basement-bounding faults, and/or differential compaction across the scarps, are considered to have had an important role in slope failure. This study highlights the importance of deeply-buried structures in the stability of slopes. It also shows how detailed examination of high-resolution data across large areas of slope instability can reveal smaller-scale multiple events. We believe that where contour-swept slopes are steep, the resulting intensification of near-bottom velocities prevents healing slope processes and as a result steep slope gradients remain steep and thus unstable.

► RBSC may have occurred as a multiphase slope collapse in at least three episodes.
► Slope and failed sediments comprise almost exclusively contourites.
► Direct relationship between RBSC scarps and basement morphology
► Focused fluid flow and differential compaction key role for instability
► Steep contour-swept slopes cannot be smoothed by slope healing and remain unstable.